writ decommission: --all removes everything writ deployed, a named removal re-converges the rest, and an implicit project is refused by name #851

Feature: #463 — writ command surface: layers, segments, adoption. Ruled 2026-09-06 (recorded on #463) alongside
the selection model in #850.

Today

writ decommission <project>... requires at least one name, reads the store's inventory, removes exactly what those
projects' runs put into effect (links unlinked, copies archived, a user-replaced target refused), and stops. It does
not re-converge the remaining projects, so a target the removed project overlaid is gone until the next deploy. Its
examples still show all as a project name from the Tuckr era.

The rulings

1. --all decommissions everything writ deployed on this machine. The bare form stays refused; destruction is
   explicit.
2. A named removal re-converges. After the named projects' entries are removed, the remaining selection is
   converged, so a target that both common and the removed project claimed reverts to common's version. apt remove re-resolves; so does this.
3. Persistent. The removed project's store entries go, so it leaves the selection (The implicit projects are common and one per configured layer repository; a repository-named project in any layer deploys unnamed #850) and no later deploy or
   upgrade brings it back unless it is named again.
4. An implicit project is refused by name — common and the repository-named projects would return on the next
   bare deploy. The refusal names the verb that retires what a layer contributes: writ repo unset <layer>.
5. No --from <layer>. A per-layer exclusion would be a fourth kind of persistent state that no repository
   records. Remove the files from the repository, or unset the layer.
6. The all example goes.

Acceptance

• writ decommission --all removes every entry in the store; writ decommission with neither names nor --all
  is refused
• after writ decommission noblefamily, a target both common and noblefamily claimed holds common's version,
  asserted end to end
• writ decommission common and writ decommission <repository-name> are refused, naming writ repo unset
• a following writ upgrade regenerates nothing of the removed project
• the command's help and docs/guides/writ/manage-environments.md state all of the above; the all example is gone

Related

#850 (the selection model), #849 (the design record), #792 (writ repo remove leaves the clone; the unset verb this
issue points at), #762 (the lifecycle surface).